Almost every cell in the body is constantly exposed to a variety of hormones, growth factors and other agents that influence the biological functions of cells. To integrate and interpret these external stimuli, complex signaling networks have evolved, which allow different types of cells to respond appropriately to their environment. In eukaryotic cells the reversible phosphorylation of proteins regulates many signaling networks that control cell growth, metabolism, differentiation, senescence, and apoptosis. Protein phosphorylation occurs principally on serine, threonine and tyrosine residues, and the phosphorylation reaction is catalyzed by a large family of protein kinases. To date many compounds that function as potent and highly selective inhibitors of "key" protein kinases have been identified, and these inhibitors have proven to be powerful tools to probe the biological and pathological actions of protein kinases. In contrast, much less is known about the biological roles of protein phosphatases. To a large extent this is due to a lack of specific or highly selective small molecule inhibitors. Having developed methods to produce large amounts of catalytically active ser/thr protein phosphatase 5 (PP5), a fluorescent assay to rapidly and reliably measure PP5 activity, and having solved the crystal structure of PP5 at high resolution (1.6 E), we now have the tools in place to develop inhibitors of PP5. The objective of this proposal is to identify a specific or highly selective inhibitor of PP5. There are two specific aims. Aim 1. Conduct a HTP-screen in conjunction with the Molecular Libraries Probe Production Centers Network (MLPCN) to identify compounds that inhibit the catalytic activity of PP5. Aim 2. Back screen compounds identified in Aim 1 against the structurally related phosphatases to determine selectivity, characterize the inhibitors, and work with members of the MLPNC for probe optimization (e.g. increase potency/specificity). The compounds produced from this effort will serve as powerful small molecule probes that will greatly aid the elucidation of the biological and pathological roles played by PP5. They may also serve as lead compounds for the development of new drugs for medical management of human cancer. PUBLIC HEALTH RELEVANCE: We have identified a protein phosphatase (PP5) that seems to be a key regulator of cell growth. Having developed methods to produce large amounts of PP5 and an assay to measure PP5 activity, we proposal is to identify a specific or highly selective inhibitor of PP5 by conducting a large scale screen in conjunction with the Molecular Libraries Probe Production Centers Network (MLPCN). The compounds produced from this effort will serve as powerful probes that will greatly aid the elucidation of the biological and pathological roles played by PP5.